The present invention relates to a method for manufacturing a silicon single crystal using a Czochralski (CZ) method.
In a conventional Czochralski (CZ) method for manufacturing a silicon single crystal, a polycrystalline silicon material is molten in a quartz crucible which has been placed in a furnace. This process forms a silicon melting solution, alternatively referred to as a silicon melt. A seed crystal is then placed in the melting solution and removed from the solution while it and the crucible are being rotated, forming a silicon single crystal below the seed crystal. In this process, the internal pressure to vacuum level in the furnace is maintained at a low level, 25 mbar or less, and has an argon gas flowing from the top downward.
As the size of the silicon single crystal increases, a larger quartz crucible is needed to melt the polycrystalline material. A larger crucible requires more heat from the heater to maintain a temperature sufficiently high to melt the polycrystalline silicon material. Heating the polycrystalline material at a higher temperature yields an increased temperature of the silicon melting solution at the crucible wall, causing an increased vapor pressure of the bubbles (e.g., SiO bubbles) present in the melting solution.
The increased vapor pressure of the SiO bubbles accelerates the evaporation of SiO and increases the quantity of SiO deposited on the furnace structures housed over the melt. The deposits drop into the silicon melting solution being pulled, contaminating the solution and causing dislocation of the crystal. This accelerated evaporation of SiO from the silicon melting solution cannot be efficiently controlled in a conventional CZ low pressure process operating at a pressure of 25 mbar or less, and where the single crystal is pulled at 100 mbar or more. A higher pressure process prevents the single crystal from the dislocation which results when the SiO evaporates, but has its own disadvantages. For example, a higher pressure process retards the release of the bubbles and impurities in the silicon melting solution from the material""s melting to completion of the single crystal pulling, but causes pinholes to form in the single crystal and increases the dislocation resulting from the bubbles and impurities.
A combination of low and high-pressure processes, in which the polycrystalline silicon material is molten at a low pressure, between 5 and 60 mbar, and the single crystal is pulled at a higher pressure, 100 mbar or more, is disclosed by Japanese Patent No. 2,635,456. Such a combination process has been used to address the problems associated with using either the high pressure process or the low pressure process for both melting and pulling. While conventional combination processes solve some of the problems of the high and low pressure processes, these combination processes have only moderately increased the single crystal yield.
Accordingly, a need exists for a method of manufacturing a silicon single crystal, which avoids the problems associated with using either a high-pressure process for both melting and pulling (e.g., formation of pinholes, and dislocation of the single crystal resulting from the bubbles and impurities) and those associated with using a low-pressure process for both melting and pulling (e.g., dislocation of the single crystal resulting from the evaporated SiO), while increasing the single crystal yield.
The present invention provides a method for manufacturing a silicon single crystal. In this method, the polycrystalline silicon material is molten in a furnace having a pressure maintained between 65 and 400 mbar, and the single crystal is pulled from a silicon melting solution at a lower pressure than that used to melt the silicon material.
Maintaining the pressure in the furnace between 65 and 400 mbar to melt the silicon material controls transformation of the quartz crucible into cristobalite and simultaneously increases the yield of the single crystal. At below 65 mbar, SiO evaporates from the silicon melting solution, causing dislocation of the single crystal and decreasing the single crystal yield. By contrast, at above 400 mbar, the inert gas flowing through the furnace may not be purged from the furnace. As a result, CO formed by the synthetic reaction between the evaporated SiO and the carbon members in the furnace remains in the furnace and contaminates the silicon melting solution, thereby increasing the number of silicon single crystal lots containing high amounts of carbon. The preferable pressure in the furnace while the silicon material is molten ranges between 120 and 200 mbar.
In the single-crystal pulling step, the pressure in the furnace should be maintained at a level not exceeding 95 mbar. At above 95 mbar, the bubbles and impurities which form from the silicon melting solution do not completely evaporate and cause formation of pinholes and dislocation of the single crystal. While the lowest acceptable pressure level in the furnace during the pulling step includes any level which is lower than that used in the material melting step and is less than 95 mbar, such level is preferably at least 10 mbar because at a pressure level less than 10 mbar, the SiO evaporates at an accelerated rate, causing dislocation of the single crystal.
The method of the present invention for manufacturing silicon single crystal is particularly suitable for a material charge of 120 kg or more and a single crystal diameter of 8 inches or more, where transformation of the crucible""s inner surface into cristobalite becomes a problem.